iMovie shows ‘modified file’ when I down load go pro movies? I then can’t work with them? Any idea what might be happening? Some work and some don’t? Seems very odd as the settings on the go pro are unchanged.

When I download from go pro into photos on my Mac, the videos appear in iMovie but then, when I try to edit, the file shows ‘modified file’. I got round this initially by revealing the file in project media but that’s now not working and the file remains ‘modified file’ with an orange line at the bottom in project media. Any ideas welcome.